The invention relates to an infusion syringe pump comprising a motor-gear unit for driving a linearly movable drive member, an injection syringe having a syringe plunger which for its actuation is in engagement with the drive member and a position detecting means for determining the position of the syringe plunger, said means having a position-defining element and a stationary absolute displacement pickup.
DE-OS 2,922,037 and DE-GM 8,520,376 disclose syringe pumps which operate with optoelectronic detection means. These detection means have the disadvantage that it is not possible to produce a continuous signal so that the accuracy of the position detection is a function of the respective code-dependent quantization of the optoelectronic elements.
US-PS 4,620,848 discloses an infusion syringe pump in which a first roller mounted stationarily on a housing is in engagement with a syringe plunger which in turn is driven by a motor, a gearing and a second roller whih is also mounted on the housing and which is drivingly connected to the motor via the gearing. The first roller, which is rotationally driven, is mounted on the housing and not on the drive member for the syringe plunger.
Furthermore, an infusion syringe pump is known from US-PS 3,701,345 in which a motor drives a plunger for expelling the fluid from a syringe cylinder. This angiographic infusion syringe pump comprises a position control system which employs a feedback voltage signal which is not associated with the syringe plunger speed but with the infusion syringe plunger position. Said feedback signal is generated by a mechanically driven potentiometer which is set in operation by the drive motor via an additional gearing. Before performing the injection the instantaneous position of the syringe plunger is sensed and the position thereof used as basis reference quantity or zero limit. The desired injection volume, corresponding voltage limits or the desired rate are predefined proportionally to the voltage by the operator.
As mentioned above, in this system a potentiometer is used to detect the instantaneous position of the syringe plunger. The position signal is compared with a control signal and the resulting difference used to operate the motor. The motor is influenced by a voltage-limiting circuit which prevents the motor delivering too much power. On return of the syringe plunger into its initial position the direction of rotation of the motor and thus of the gearing is reversed. In such infusion syringe or injection pumps in which the injection or syringe plunger is returned to its initial position in dependence upon the motor-gear unit by reversal of the direction of rotation of the motor it is admittedly possible to determine at any time an instantaneous absolute position value of the syringe plunger; however, the construction of the position control system is made complicated by the necessity of an additional gearing transmission for the position control system. The presence of an additional gearing transmission can moreover introduce a source of error into said control system because inaccuracies or errors of the additional gearing transmission can give rise to a deviation of the actual position of the syringe plunger in the syringe cylinder from the position information supplied by the position control system.
In addition, such a pump is complicated in manipulation because the entire drive arrangement must be moved to the initial state when a new (filled) syringe is to be inserted.
In other known infusion injection or syringe pumps in which the injection or syringe plunger can be returned to its zero position independently of the motor-gear unit, such as for example described in DE-OS 3,428,655, it would be desirable to have such a position value determination.
According to the invention the problem is solved in that the position-defining or position-indicating element is rigidly connected to the linearly movable drive member, and that the position-defining element along its path of movement corresponding to the displacement direction of the drive member is electrically or magnetically coupled to the absolute displacement pickup.
In a preferred embodiment the absolute displacement pickup is constructed as linear potentiometer.
In addition, the absolute displacement pickup may be a capacitive or magnetic position-defining element.
Furthermore, the absolute displacement pickup is connected to an evaluating unit.
Due to the preferably rigid connection of the position-pickup element to the drive member driving the syringe plunger the possible source of error of the defective auxiliary gear in US-PS 3,701,345 can be eliminated. Consequently, due to the direct tapping off of the instantaneous plunger position extensive freedom from error can be ensured.
Infusion syringe pumps which can move the syringe plunger into any desired initial position independently of the motor-gear unit can be provided with a drive spindle leading to the motor-gear unit and a drive member which by an actuating element can be disengaged from the drive spindle by rotation about its longitudinal axis and thus pulled forwardly or rearwardly independently of the motor-gear unit, as is the case in DE-OS 3,428,655. By reengagement of the drive member the latter can be locked again in any desired position.
Due to the fixed connection the position-indicating or position-pickup element is displaced with the drive member and correlated thereto represents the position of the drive member and thus the syringe plunger.
The invention will be explained in detail hereinafter with the aid of an example of embodiment.